Examples of known processes for the microbial production of .alpha.-hydroxy acids include a process in which glycolic acid, lactic acid, .alpha.-hydroxyisobutyric acid and the like are produced from corresponding .alpha.-hydroxynitrile compounds by strains belonging to the genusCorynebacterium (cf. JP-A-61-56086; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a process in which lactic acid, glycolic acid and the like are produced from corresponding .alpha.-hydroxynitrile compounds by strains belonging to the genus Bacillus, Bacteridium, Micrococcus or Brevibacterium (cf. U.S. Pat. No. 3,940,316 and JP-B-58-15120; the term "JP-B" as used herein means an "examined Japanese patent publication"), a process in which lactic acid, .alpha.-hydroxyisobutyric acid, mandelic acid, .alpha.-hydroxybutyric acid, .alpha.-hydroxyvaleric acid, .alpha.-hydroxy-.alpha.-phenylpropionic acid, .alpha.-hydroxy-.alpha.-(p-isobutylphenyl)- propionic acid and the like are produced from corresponding .alpha.-hydroxynitrile compounds by strains belonging to the genus Pseudomonas, Arthrobacter, Aspergillus, Penicillium, Cochliobolus or Fusarium (cf. JP-A-63-222696), a process in which .alpha.-hydroxy-.beta., .beta.-dimethyl-.gamma.-butyrolactone is produced from a corresponding .alpha.-hydroxynitrile compound by strains belonging to the genus Arthrobacter, Aspergillus, Bacillus, Bacteridium, Brevibacterium, Cochliobolus, Corynebacterium, Micrococcus, Nocardia, Penicillium, Pseudomonas or Fusarium (cf. JP-A-64-10996), a process in which mandelonitrile, mandelamide or a substitution product thereof is asymmetrically hydrolyzed by strains belonging to the genus Alcaligenes, Pseudomonas, Rhodopseudomonas, Corynebacterium, Acinetobacter, Bacillus, Mycobacterium, Rhodococcus or Candida (cf. U.S. Pat. No. 5,283,193 and JP-A-2-84198), a process in which .alpha.-hydroxyisobutyric acid is produced from .alpha.-hydroxyisobutyronitrile by strains belonging to the genus Rhodococcus, Pseudomonas, Arthrobacter or Brevibacterium (cf. JP-A-4-40897), a process in which a predominant amount of R(-)-mandelic acid or a derivative thereof is directly produced from a racemic compound of mandelonitrile or a derivative thereof by strains belonging to the genus Pseudomonas, Alcaligenes, Acinetobacter, Caseobacter, Nocardia, Bacillus, Brevibacterium, Aureobacterium or Rhodococcus (cf. U.S. Pat. No. 5,223,416 and 5,296,373, JP-A4-99495, JP-A-4-99496, JP-A-4-218385 and JP-A-5-95795) and a process in which a predominant amount of D- or L-lactic acid is directly produced from DL-lactonitrile by strains belonging to the genus Enterobacter, Arthrobacter, Caseobacter, Brevibacterium, Aureobacterium, Escherichia, Micrococcus, Streptomyces, Flavobacterium, Aeromonas, Nocardia, Mycoplana, Cellulomonas, Erwina, Candida, Pseudomonas, Rhodococcus, Bacillus, Alcaligenes, Corynebacterium, Microbacterium or Obesumbacterium (cf. U.S. Pat. No. 5,234,826, JP-A-4-99497 and JP-A-5-21987).
Examples of known processes for the microbial production of .alpha.-hydroxyamides include a process in which the corresponding amides are produced from lactonitrile, hydroxyacetonitrile, .alpha.-hydroxymethylthiobutyronitrile and the like by strains belonging to the genus Bacillus, Bacteridium, Microciccus or Brevibacterium (cf. U.S. Pat. No. 4,001,081 and JP-B-62-21519) and a process in which .alpha.-hydroxy-4-methylthiobutylamide is produced from .alpha.-hydroxy-4-methylthiobutyronitrile by strains belonging to the genus Rhodococcus, Corynebacterium, Pseudomonas, Arthrobacter or Alcaligenes (cf. JP-A-4-40899), as well as a report stating that lactamide is accumulated as an intermediate when lactonitrile is hydrolyzed into lactic acid by a strain belonging to the genus Corynebacterium (Grant, D. J. W., Antonievan Leauwenhoek, vol. 39, p. 273, 1973).
However, it is known that .alpha.-hydroxynitrile compounds partially dissociate into the corresponding aldehydes and prussic acid in polar solvents, having a certain dissociation constant depending on each compound (cf. V. Okano et al., J. Am. Chem. Soc., vol. 98, p. 4201, (1976)). In addition, since aldehydes generally are able to inactivate enzymes by binding to the protein (cf. Chemical Modification of Proteins, G. E. Means et al., Holden-Day, 125, 1971), they cause a problem when an .alpha.-hydroxynitrile compound is subjected to enzymatic hydration or hydrolysis, because the aldehyde formed by the dissociation equilibrium inactivates the enzyme within a short period of time, thus making it difficult to obtain .alpha.-hydroxy acid or .alpha.-hydroxyamide in a high concentration with high productivity.
As a means for overcoming these problems, a process has been proposed in which a sulfite ion, an acid sulfite ion or a dithionite ion is added to a reaction system to effect formation of a reversible complex with the dissociated aldehyde, thereby stabilizing the reaction through sharp reduction of the free aldehyde level in the reaction system (cf. U.S. Pat. No. 5,326,702 and JP-A-5-192189).
However, the dissociation equilibrium constant of complexes with sulfite ions and the like greatly varies depending on the type of aldehyde. Because of this, when an aldehyde which forms a complex having an extremely low dissociation equilibrium constant is used as the target, addition of sulfite ions and the like causes a sharp decrease in the concentration of .alpha.-hydroxynitrile in the reaction system to a level lower than the necessary concentration for the enzyme reaction and therefore results in termination of the enzyme reaction. This observation shows that toxicity of all aldehydes cannot easily be reduced by merely adding the aforementioned sulfite ions and the like (The Chemistry of the Carbonyl Group, vol. 2, ed. by Jacob Zabicky, 1970, Interscience Publishers).